Co-application of straw incorporation and biochar addition stimulated soil N2O and NH3 productions

Nitrous oxide (N2O) and ammonia (NH3) volatilization (AV) are the major pathways of nitrogen (N) loss in soil, and recently, N2O and NH3 mitigation has become urgently needed in agricultural systems worldwide. However, the influence of straw incorporation (SI) and biochar addition (BC) on N2O and NH3 emissions are still unclear. To fill this knowledge gap, a soil column experiment was conducted with two management strategies using straw ‐ straw incorporation (S1) and straw removal (S0) ‐ and four biochar application rates (0 (C0), 15 (C1), 30 (C2), and 45 t ha−1 (C3)) to evaluate the impacts of their interactions on N2O and NH3 emissions. The results showed that NO3−−N concentration and pH was the major contributors to affect the N2O and NH3 losses. Without biochar addition, N2O emissions was decreased by 59.6% (P<0.05) but AV was increased by 97.3% (P<0.05) under SI when compared to SR. Biochar was beneficial for N2O mitigation when straw was removed, but increased N2O emission by 39.4%−83.8% when straw was incorporated. Additionally, biochar stimulated AV by 27.9%−60.4% under S0 and 78.6%−170.3% under S1. Consequently, SI was found to significantly interact with BC in terms of affecting N2O (P<0.001) and NH3 (P<0.001) emissions; co-application of SI and BC promoted N2O emissions and offset the mitigation potential by SI or BC alone. The indirect N2O emissions caused by AV, however, might offset the reduction of direct N2O caused by SI or BC, thus leading to an increase in overall N2O emission. This paper recommended that SI combined BC at the amount of 8.2 t ha−1 for maintaining a lower overall N2O emission for future agriculture practices, but the long-term impacts of straw incorporation and biochar addition on the trade-off between N2O and NH3 emissions and reactive N losses should be further examined and assessed.


Introduction
Globally, fertilized cropland are the major source of nitrogen pollutants, such as ammonia (NH 3 ) and nitrous oxide (N 2 O) [1].N 2 O has 273 times greater global warming potential than carbon dioxide when assessed over a 100−year time scale, and also accelerates ozone depletion [2].Intensively used agricultural soil have been identified to be the main source of N 2 O emission, accounting for 60% of total anthropogenic-caused release at approximately 3.5 Mt N 2 O −N per year [3].
NH 3 volatilization (AV) following nitrogen (N) fertilizer application is a major pathway of soil N loss from cropping systems worldwide [4].It has been estimated that agriculture contributes 80%−90% of the total NH 3 emitted in many countries, and globally, NH 3 emissions increased by 128% over the last four decades [5].Additionally, indirect greenhouse gas (GHG) emissions induced by NH 3 losses have been found to be up to 5%−12% [6].Recently tackling the trade-off between N 2 O and NH 3 emissions has been a hot-spot [7], and agricultural soil is increasingly being scrutinized for its contribution to air quality degradation [8].Moreover, many studies have focused on how to mitigate N 2 O and NH 3 losses to meet the goal of increasing nitrogen use efficiency, decreasing environmental risks for future intensive agriculture.
Northern China, the most important and intensified crop production region in the country, is an area of annual winter wheat (Triticum aestivum L.)/summer maize (Zea mays L.) rotation systems.Unfortunately, intensive agricultural systems are still inefficient in N fertilizer use; around 50%-70% of fertilizer N is lost to the environment [9].Another striking feature of intensive agriculture is a large amount of crop straw production.In China, straw production exceeds more than 10 9 Mg per year, accounting for 25% of global production [10].Crop straw incorporated in the field improves soil fertility and reduces the severe air pollution caused by burning of straw [11], minimizes negative environmental impacts [12], increases soil C sequestration [13][14][15] and enhances cereal crop yields [16], which has been widely recommended as an environmentally friendly strategy in agricultural ecosystem [15].Additionally, straw incorporation has been shown to induce net N immobilization, along with reducing NO 3 − leaching and N 2 O and NH 3 emissions [17].However, the results of previous studies that assessed the influence of straw incorporation on N 2 O emissions were found to be inconsistent, showing positive, negative, and neutral effects.For example, Liu et al. [15] reported that N 2 O emission was increased by 8.3% in upland soils but decreased by 15.2% in paddy soils when straw was incorporated, mainly because of a mineralizable-N substrate for N 2 O generation through nitrification process and reduced oxygen availability in the soil profile which favored N 2 O production through denitrification [18][19][20].Moreover, previous studies have shown that straw incorporation significantly increased soil NH 4 + −N concentration and induced 45.7% more AV [21].This is because when straw was incorporated, the ratio of soil N immobilization was lower than that of N mineralization [21,22].
Straw can be further derived to yield a highly stable biomass-pyrolysis product known as biochar.As a new approach to returning agricultural waste to the field, straw-derived biochar application can affect both N 2 O and NH 3 losses through increasing soil carbon sequestration and reducing carbon mineralization and non−CO 2 emissions from the biochar itself [23].Biochar was beneficial to change the N 2 O emission and NH 3 volatilization, reduce soil organic matter mineralization [24], and improve root biomass, yield, water use efficiency, and soil microbial activities [25].Previous studies have shown that biochar could reduce current anthropogenic CO 2 −eq emissions by 12% without endangering food security [26].Biochar addition can change soil physical and chemical properties, such as increasing the soil carbon content, C/N ratio, pH, soil water holding capacity, and reducing NH 4 + −N and NO 3 − −N leaching [27], thus affecting the N 2 O emissions and AV from agricultural soil.For example, Feng et al. [28] have found that soil N 2 O emission was increased by 7.7%−21.2%,but Yang et al. [29] reported that soil N 2 O emission was decreased by 46.6% under biochar addition.Feng et al. [28] reported that biochar treatments recorded 9.9%−70.9%higher AV compared with control mainly due to the increase of soil pH after biochar addition.Sun et al. [30] has found that biochar addition significantly decreased N leaching by 11.6%−29.7%,but not significantly affected AV when 0.5% and 1% biochar amended and increased by 25.6%−53.6%higher AV when 2% and 4% biochar amended.Therefore, the conclusions about the effect of biochar addition on AV were inconsistent, however, and mainly depend on the extent of soil pH change, the ammonium retention capacity, and addition rate of biochar [31].
This study aimed to assess if straw incorporation combined with biochar application could be an efficient measure to reduce N 2 O and NH 3 losses.An indoor soil column experiment was conducted to investigate the comprehensive influences of biochar (derived from maize straw) addition and maize straw incorporation on soil N 2 O and NH 3 losses.The objectives of this study were to determine the response of N 2 O and NH 3 losses to SI and BC, to explore the trade-off between N 2 O and NH 3 emissions under SI and BC, and to establish the soil conditions to identify measures for simultaneous reduction of N 2 O and NH 3 losses.

Background information and soil column installation
A soil column experiment was conducted from April to May 2021 at the State Key Laboratory of North China Crop Improvement and Regulation, Hebei Agricultural University, China.PVC pots were used (20 cm diameter × 60 cm height).Each column was equipped with a static chamber on the top for gas samples collection (Fig 1A).The experimental soil was collected from 0−10 cm, 10−20 cm, and 20−40 cm of depth from the soil profile in a wheat-maize rotation field located in Sanfenchang field station, Hebei Province, China (38˚51 0 30˝N, 115˚28 0 52˝E).The soil samples were air-dried and passed through a 2−mm sieve, then repacked to soil columns in the same order and at the same bulk density.The soil properties of surface layer (0 −20 cm) were shown in Table 1.

Experimental treatments and managements
In this study, the two factors considered were straw and biochar managements.Biochar was derived from maize straw in a continuous slow pyrolysis system at 550˚C.The properties of the biochar used in this experiment were shown in Table 1.Two straw management strategiesstraw incorporation (S1) and straw removal (S0), and four biochar addition rates (0 [C0], 15 [C1], 30 [C2], and 45 t ha −1 [C3]) were conducted.The experimental design included eight treatments: S0C0, S0C1, S0C2, S0C3, S1C0, S1C1, S0C2, and S1C3, arranged in three replicates each into 24 soil columns total.The straw, biochar, and basal fertilizer were homogeneously mixed with the surface soils (0−20 cm).
The wheat was transplanted on April 7, 2021 with a basal fertilizer of 50% total N (i.e., 105 kg N ha −1 ), 90 kg P 2 O 5 ha −1 , and 120 kg K 2 O ha −1 .The topdressing fertilizer was applied on May 4, 2021.During the wheat-growing season, all experimental columns were irrigated at the amount of 75 mm in three times (Fig 1).

Measurements of N 2 O and NH 3
N 2 O gas samples were collected through a closed chamber method, and were analyzed using a gas chromatograph (Agilent 7820A, Agilent Technologies Inc., US) equipped with an electron capture detector using the DN−CO 2 method.For the five gas samples analyzed, N 2 O flux was calculated by a linear method using Eq 1 as follows: where F = N 2 O flux, μg N m −2 h −1 ; M = gas molar mass, g�mol −1 ; V 0 = gas volume in the standard state, 22.41×10 −3 m 3 ; T = temperature on the sampling day,˚C; P = air pressure on the sampling day, hPa; P 0 = air pressure on the standard day, 1013 hPa; H = height, cm; and dC t / d t , the linear or non-linear slope of the N 2 O concentration change over time in the static chamber.
Daily NH 3 volatilization fluxes were measured by a continuous airflow enclosure method using a Plexiglas chamber (20 cm inner diameter and 20 cm height).NH 3 emitted from soil was absorbed by dilute H 2 SO 4 (0.01 M) solution.The NH 4 + −N concentration of the resulting solutions was then determined by a continuous flow analyzer (TRAACS2000, Norderstedt, Germany), and the NH 3 volatilization fluxes were calculated according to Eq (2): where   The overall N 2 O emission was defined as direct N 2 O plus indirect N 2 O from NH 3 volatilization, in which the indirect N 2 O emission factor from NH 3 was defined as 1% according to Wu, et al. [7].

Auxiliary measurements
The amount of irrigation water applied was manually recorded at each occurrence.During N 2 O and NH 3 sampling, the air temperature, soil temperature (0−5 cm) and soil water content in each treatment were simultaneously observed and recorded.Gravimetric water content was measured by drying the soil at 105˚C for 24 h.Water-filled pore space (WFPS) was calculated according to the Eq (3) The second portion was used for soil organic matter (SOM, digestion with H 2 SO 4 −K 2 Cr 2 O 7 and titration), total N (TN, H 2 SO 4 -mixed accelerator-distillation using the Kjeldahl method), and pH determination.

Calculations and data analysis
All data collected were analyzed using one-way analysis of variance (ANOVA) in SPSS Statistics 22.0 (SPSS Inc., Beijing, China).Means of AV monitored by different methods were compared followed the least significant difference (LSD) test at the 5% level of probability.The effects of different straw and biochar managements and their interactions on soil N 2 O and NH 3 losses, soil conditions under different soil layers were analyzed by the two-way ANOVA.Graphs were produced with Origin 9.1.The RDA was estimated by Canoco 5 (version 5.02) software.

Works approval
All works were conducted and permitted by Hebei Agricultural University.This article does not contain any studies with human participants performed by any of the authors.

Soil temperature, WFPS and chemical parameters
Under the same straw practice, no significant difference among these four biochar treatments was found (Figs 4 and 5).The WFPS of S1C0 was 20.5% (P<0.05)lower that of S0C0 (  −N content, however, was only found in 0−20 cm layer.The high N 2 O emission caused by biochar addition was mainly due to its negative influence on soil pH and the increase in soil moisture and SOM, which was particularly noticeable in the 0−10 cm soil layer.In addition, SI contributed to SOM increase (P<0.05) in surface soil, leading to high NH 3 volatilization (P<0.05).

Effect of straw incorporation and biochar on soil N 2 O, NH 3 , and other chemical parameters
Biochar was beneficial to increase soil moisture (P<0.05)(particularly in the 0−20 cm soil layer) as well as N 2 O mitigation (P<0.05)under SR condition.No significant relationships were found between NH 3 emission and the measured variables.

Response of N 2 O emissions to straw incorporation and biochar addition
Soil N 2 O production is stimulated by native soil N (background), fertilizer N, and the priming effect [32]; the influence of field management strategies on soil N 2 O emissions is mainly due to the impacts of fertilizer N and the priming effect [33].In previous studies, SI was found to be beneficial to increase the concentration of SOC and total N, and C/N ratio through available N release or soil N immobilization processes [19].In this study, a 59.6% reduction in N 2 O emissions was found under SI without biochar addition (Fig 3).Additionally, C/N ratio, SOM and NO 3 -concentration was positively correlated to soil N 2 O emission, and the soil NO 3 -concentration was the major contributor (accounting for 51.2%) to soil N 2 O and NH 3 losses, and the pH was the next one through a redundancy analysis (Fig 7).Eventhough, no significant influence of SI on SOM was found, our previous study have demonstrated that SI has a positive impact on C sequestration [34].Therefore, SI is regarded as an important way to affect soil N 2 O emission [35].Straw managements showed a significant effect on SOM and soil NO 3 -  concentration in 0−10 cm soil layer (Table 1).Some studies have found that N 2 O emission was positively correlated with soil NO 3 -and SOM concentration [35,36].However, no similar results were observed in our study, which indicated that changes in available N might not be the main factor affecting soil N 2 O productions.As previous studies reported that transient subsequent microbial N immobilization might have occurred with straw-C inputs [23,37].Chen et al. [36] has found that maize straw incorporated into soil would decrease seasonal N 2 O emissions by decreasing the contribution of denitrification to N 2 O emissions and  through decreasing the abundance of N functional genes, thus caused the higher of soil NO 3 concentration under SI than SR.
Biochar derived by maize straw was also expected to have potential to improve soil nutrient retention.Some previous studies have also observed significant reductions in soil N 2 O emissions after biochar addition, potentially because of the increase of soil organic C contcentration and C/N ratio and adsorption of NH 4 + −N by biochar [30].A high soil C/N ratio promoted soil N assimilation and immobilization, and resulted in the consumption of by nitrifiers and denitrifiers [24].Additionally, in northern China, a reduction in N 2 O emissions was also found due to the reduction process by nitrifier denitrification (by 74%) and heterotrophic denitrification (by 58%) [24] with the increase in nosZ gene prevalence following biochar application [31].In this study, compared with S0C0, adding biochar significantly reduced N 2 O production from the soil (approximate decrease of 35.0%−54.7%),showing a binomial relationship (S3 Fig) .Unfortunately, no coordinated mitigation potential for N 2 O emissions was found for coapplication of straw and biochar, but promoted by 39.4%−83.8%(P<0.05) as comparison to S1C0 (Figs 3 and S1).In this study, the significant influence of SI, biochar addition, and their interactions were found on N 2 O emissions (Table 1).These results might be explained that straw decomposition was accelerated and thus increased the contcentration of surface soil organic matter when straw was incorporated and biochar was applied [24,38].The co- application of straw and biochar also likely significantly promoted the positive priming effect on soil organic N mineralization, increasing NH 4 + −N concentrations in the 10−20 cm soil layer [39].The treatment also increased the soil NO 3 − −N concentration in the 0−10 cm soil layer by promoting nitrification, ultimately resulted in the increase of soil N 2 O emissions [24].

NH 3 emissions response to straw incorporation and biochar amendment
In this study, NH 3 volatilization was increased by 97.3% under SI, and significant correlated to SOM, TN, and NH 4 + concentrations (Figs 3 and 7).Incorporation of maize straw with high C/ N ratio could stimulate soil microbial activity, as previously discussed, enhancing straw decomposition and urea hydrolysis [21,22]; this likely increased the NO 3 − concentrations in the 0−10 cm soil layer (Table 1), indicating that nitrification was promoted and contributed to NH 3 reduction.Higher concentration of SOM and TN, and pH were found under SI (Fig 8) . Consequently, the increased potential for NH 3 volatilization through the hydrolysis of urea may have offset its reduction potential through the nitrification process, causing the observed increase of AV [40].A significant correlation was found between the amount of biochar addition and pH/NH 4 + in the 0−10 cm soil layer (Table 1).Additionally, biochar would increase AV by 27.9%−60.4%under SR, and a a binomial relationship was found between the amount biochar addition and AV (S4 Fig), which was in line with Sun et al. [30] and Feng et al. [28].Through a redundancy analysis, we found pH was a main contributor (accounting for 14.3%) to NH 3 emission (Fig 7).As Liu et al. [6] reported that the increase of AV induced by biochar might be caused by the strong increase of soil pH, thus promoted the hydrolysis of urea and inhibited the nitrification processes (especially in the 10−40 cm soil layer), thus increased soil NH 4 + concentration [41].
AV decreased gradually with the increase of the amount of biochar addition; this was mainly due to the high NH 4 + /NH 3 absorption/immobilization by biochar offsetting the NH 4 + hydrolyses from urea [42].When the co-application of straw and biochar, the AV increased by 78.6%−170.3%(P<0.01)(Fig 3 and Table 1).Previous studies have shown that biochar was beneficial for straw decomposition and SOM mineralization, then increasing NH 4 + concentrations [24,38] and resulting in a positive effect on AV (S4 Fig).

Tackling the trade-offs between NH 3 and N 2 O emission using straw incorporation and biochar addition
Recently, approaches to tackle the possible trade-off between N 2 O and NH 3 emissions in croplands have been studied [40,41].In fertilized fields, many factors have inconsistent impacts on NH 3 volatilization and N 2 O emission [3], implying that a trade-off is required in decisions surrounding practical management of straw incorporation and biochar addition.As evidenced by Fig 3, lower N 2 O emissions (straw effects vs. biochar effects: 59.6% vs. 97.3%)and higher NH 3 volatilization (straw effects vs. biochar effects: 35.0%−54.7% vs. 27.9%−60.4%)were achieved under SI or biochar addition, indicating that either the single-or co-application of these compounds was unable to achieve the win-win goal of both N 2 O and NH 3 mitigation [24].Additionally, the indirect and direct N 2 O emissions were accounted for 13.8%−49.0%and 67.6%−131.1% of overall N 2 O emission under S0 and S1 treatment, respectively (S5 Fig) .For the overall N 2 O emission, both negative (for biochar amount lower than 8.2 kg ha −1 ) and positive effects were observed under SI (Fig 9).Therefore, using SI or biochar addition could lead to high NH 3 volatilization when urea was applied, and stimulated the indirect N 2 O emissions that offset the mitigation potential of direct N 2 O reduction [40,41].Because of this, when biochar is applied in cropland, the influence of straw managements on soil N 2 O and NH 3 emissions should be further assessed for future sustainable agriculture.Additionally, when straw was incorporated, biochar applied at the amounts of 8.2 kg ha −1 are recommended based on this study for overall N 2 O mitigation, though further work remains to achieve better reduction.

Conclusions
Fertilized cropland is a major source of ammonia (NH 3 ) and nitrous oxide (N 2 O) losses.This study demonstrated that NO 3 − −N concentration and pH was the major contributors to affect the N 2 O and NH 3 losses.Single straw incorporation (SI) or biochar addition could mitigate N 2 O emission, but co-application contributed to the increases of N 2 O emission.Eventhough, the single-or co-application of SI and biochar both induced NH 3 volatilization (AV).The effects of SI and biochar on N 2 O and NH 3 emissions could be partly ascribed to the straw decomposition, urea hydrolysis, and nitrification.Additionally, the indirect N 2 O emission induced by AV offset the mitigation potential for direct N 2 O emissions when straw incorporated or biochar applied.Overall, the co-application of SI with BC at the amount of 8.2 t ha −1 is recommended to successfully maintain a lower overall N 2 O emission to tackle the trade-off between N 2 O and NH 3 emissions.For future agriculture, the long-term impacts of straw incorporation and biochar addition on the trade-off between N 2 O and NH 3 emissions and reactive N losses should be further examined and assessed.
of the dilute H 2 SO 4 solution, mL; A = crosssectional area of the capture device, m 2 ; and t = successive capture time, h.The cumulative N 2 O emission and NH 3 volatilization were estimated by summing the daily mean fluxes, and the daily fluxes of non-measurement days were estimated by interpolating linearly between sampling dates.
N 2 O emissions (Figs 2A and 2C and S1).When straw was incorporated, the highest N 2 O flux was found under C2 treatment (265.2 μg N m −2 h −1 ) at the top-dressing (Fig 2A).Biochar significantly increased the soil N 2 O flux peaks, especially in the first four days after fertilization, thus significantly increased N 2 O emissions (Fig 3).The cumulated N 2 O emissions were in the order of C3>C1>C2>C0, in which the N 2 O emission of C3, C1, and C2 were 83.8% (0.66 kg N ha −1 , P<0.05), 56.9% (P<0.05), and 39.4% (P<0.05)higher than that of C0, respectively.When straw was removed, the highest N 2 O flux was found under C0 treatment (794.0 μg N m −2 h −1 ) at the top-dressing (Fig 2C).In this condition, biochar application significantly decreased soil N 2 O flux peaks and contributed to N 2 O mitigation by 35.0%−54.7%under straw removal (SR).Additionally, N 2 O emission under S1C0 was 59.6% lower than that of S0C0 (0.89 kg N ha −1 ).NH 3 fluxes and cumulative emissions Daily NH 3 fluxes exhibited apparent temporal patterns, and the flux peaks were occurring on the first 1−7 days after fertilization.Additionally, the NH 3 volatilization (AV) was stimulated by

Fig 3 −−
total N, and SOM varied in different soil layers under different treatments (Fig 6).In surface soil (0−10 cm), SI and BC were both beneficial for the increase of soil NO N concentration.However, the highest soil NO 3 − −N concentration was found in C1, and gradually decreased along with the increase of biochar addition amount under SR.In the 10−40 cm soil layer, soil NO 3 − −N was only influenced by the addition amount of biochar.SI only contributed to the increase of NH 4 + −N concentration in surface soil (0−10 cm).In the 20−40 cm soil layer, soil NH 4 + −N concentration under SR was increased by 81.1% (P<0.05) in comparison to SI. BC highly affected the influence of soil TN concentration under SI; for instance, in the 0−10 cm layer, TN was increased by 40.4% (P<0.05)under SI as compared to SR under C3 (45 kg C ha −1 ).Additionally, biochar addition further contributed to the increase of SOM especially in the 0−10 cm and 20−40 cm soil layers.

Soil NO 3 −− 3 −
N concentration and pH were the main contributor (accounting for 51.2%, 14.3%, respectively) to affect soil N 2 O emission and NH 3 volatilization (Fig 7).SI significantly interacted with biochar addition in terms of affecting N 2 O and NH 3 emissions, soil pH and NO in 0−10 cm soil layer, and SOM in 20−40 cm soil layer (Table 2).Under SI, biochar application particularly stimulated N 2 O and NH 3 losses (Fig 8).The soil moisture and SOM significantly increased (P<0.05)along with the increase of biochar addition amount in 0−40 cm soil layer.The positive influence of biochar on NO 3 − −N and NH 4 +

Fig 6 . Soil NO 3 −−N, NH 4 +
Fig 6.Soil NO 3 − −N, NH 4 + −N, total N, and organic matter content in different soil layers under different treatments.Error bars denote standard errors.The different letters in the same soil layer indicate a significant difference (P<0.05) with Turkey's multiple range test of different amounts of biochar addition either under straw incorporation or not.Definitions of C0, C1, C2 and C3 are given in caption of Fig 2.